This invention relates to a driving circuit for a semiconductor laser which is applied to an optical disk recording/reproducing apparatus using a recordable compact disk (CD-R), digital versatile disk (DVD-RAM), magneto-optic disk (MO), for example.
As the laser driving circuit applied to the above type of apparatus, an automatic power control (APC) system and automatic current control (ACC) system are provided.
FIG. 12 shows a conventional apparatus of APC system used for an MO, for example. The apparatus includes a laser driving circuit 101, a laser diode (LD) 102 constructed by a semiconductor laser, lenses 103, 105, 106, 108 constructing an optical system, a beam splitter 104, a 4-segment sensor 107, for example, for detecting reflected light from an optical disk 110 at the data readout time, and a photodiode 109 for detecting an output signal from the laser diode 102 and feeding the same back to the laser driving circuit 101. The laser driving circuit 101 controls the amount of emission light of the laser diode 102 according to a signal supplied from the photodiode 109.
FIG. 13 shows a conventional apparatus of ACC system used for CD-R or DVD-RAM, for example. The apparatus includes a laser driving circuit 111, a laser diode (LD) 112, lenses 113, 115, 116 constructing an optical system, a beam splitter 114, a 4-segment sensor 117, for example, for detecting reflected light from an optical disk 119 at the data readout time, a photodiode 118 used as a photodetector for detecting an output light amount of the laser diode 112, an amplifier 120 for amplifying an output signal of the photodiode 118 and a digital signal processing circuit 121 for processing an output signal of the amplifier 120 and supplying the processed signal to the laser driving circuit 111. The digital signal processing circuit 121 includes an analog/digital converter (A/D) 121a for converting an output signal of the amplifier 120 into a digital signal, a microprocessor unit (MPU) 121b for processing an output signal of the analog/digital converter 121a, and a digital/analog converter (D/A) 121c for converting an output signal of the microprocessor unit 121b into an analog signal. The laser driving circuit 111 controls the amount of emission light of the laser diode 112 according to a signal supplied from the digital signal processing circuit 121 by use of a current value thereof.
In the case of APC system shown in FIG. 12, a plurality of pulse signals are generated for each pit in order to form pits of good shape in the optical disk. The laser diode is driven by the pulse signals. Therefore, the laser diode 102 is driven by a high-frequency pulse signal. As a result, it is required for the photodiode 109 which monitors the light emission output of the laser diode 102 to have a wide-band characteristic of 500 MHz, for example. Further, a high-precision lens is used as the lens 108 for guiding the laser light to the photodiode 109. On the other hand, in the case of ACC system, the high-speed response in comparison with the APC system is not required. Therefore, the photodetector and optical system can be made simple in construction.
However, in the conventional ACC system shown in FIG. 13, in the laser driving circuit 111 constructed by a semiconductor integrated circuit, variations occur in resistances of resistors constructing the circuit. Further, the resistance fluctuates depending on the temperature. In addition, the transistor characteristic also fluctuates depending on the temperature. Therefore, it is difficult to stabilize the laser output with high precision.